It was part engineering marvel, part video game challenge Friday as crews painstakingly worked to lower a box the size of a house over the ruptured oil well that has spewed an estimated 3 million gallons of crude into the sea.
Camera-equipped underwater robots operated by joystick from the surface labored to maneuver the 100-ton concrete-and-steel vault into place so it could eventually capture the oil and funnel it up to a tanker.
It was an intrepid attempt to defuse an environmental crisis that has been unfolding since a deepwater drilling platform exploded April 20, sending toxic oil toward a shoreline of marshes, shipping channels, fishing grounds and beaches. Eleven workers were killed in the accident.
The slow-moving drama was playing out 50 miles from Louisiana's coast, requiring great precision and attention to detail. It took about two weeks to build the 40-foot box, and the effort to lower it by crane and cable to the seafloor began late Thursday night. It could be Saturday before the vault is on the bottom.
"We are essentially taking a four-story building and lowering it 5,000 feet and setting it on the head of a pin," BP spokesman Bill Salvin told The Associated Press.
The task became increasingly urgent as toxic oil crept deeper into the bays and marshes of the Mississippi Delta.
A sheen of oil began arriving on land last week, and crews have been putting out floating barriers, spraying chemical dispersants and setting fire to the slick to try to keep it from coming ashore. But now the thicker, stickier goo — arrayed in vivid, brick-colored ribbons — is drawing ever closer to Louisiana's coastal communities.
The box must be properly aligned over the main leak, after which robots will secure it and attach the plumbing that will carry the oil up to the surface. It could be collecting as much as 85 percent of the oil gushing into the Gulf by Sunday.
But there are untold risks and unknowns: The approach has never been tried at such depths, where the water pressure is enough to crush a submarine, and any wrong move could damage the leaking pipe and make the problem worse. The seafloor is pitch black and the water murky, though lights on the robots illuminate the area where they are working.
If the box works, another one will be dropped onto a second, smaller leak at the bottom of the Gulf.
At the same time, crews are drilling sideways into the well in hopes of plugging it up with mud and concrete, and they are working on other ways to cap it.
The well has been spewing about 200,000 gallons a day in the nation's biggest oil spill since the nearly 11 million gallons lost in the Exxon Valdez disaster in Alaska in 1989.
The cause of the blast has not been determined, but investigators have been focusing on the so-called blowout preventer. Federal regulators told The Associated Press that they are going to examine whether these last-resort cutoff valves on offshore oil wells are reliable.
At Hopedale, a fishing community in St. Bernard Parish, La., that has been a staging area for efforts to protect inlets and bayous, Louisiana Gov. Bobby Jindal stepped out of a helicopter and held aloft a tennis ball-size hunk of tarry oil he said a fisherman had retrieved near the mouth of the Mississippi River.
Oil was reported moving west of the Mississippi toward fishing and resort villages on the Louisiana coast.
After a flyover, Jindal described the orange and brown goo surrounding Louisiana's Chandeleur Islands as resembling "a ring around your bathtub."
BP plans to sell the petroleum it recovers after separating out the large amounts of natural gas and seawater — something that industry experts said should not present much of a problem.
"That's something they do for every oil well," said Don Van Nieuwenhuise, director of petroleum geoscience programs at the University of Houston. "They'll refine it and crack it and everything, and by the time it gets in your gas tank, you'll never even know it was in the water."
The oil's planned destination, BP's Texas City, Texas, refinery, has its own checkered history. An explosion there in 2005 killed 15 people and injured 170. Regulators last October hit BP with a record $87 million fine for safety violations.
Associated Press writers Cain Burdeau, Vicki Smith and Ray Henry in Louisiana, and Michael Graczyk in Houston contributed to this report.
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